DOCBOOK-APPS: looking for XML editor that supports outline mode

["Robert P. J. Day" <rpjday at mindspring dot com>]

  i might have gotten responses when i posted about this once
upon a time, but somehow, i can't find those responses, so here
goes again.

  i would dearly love to find an XML editor that supports
outline mode, as basic emacs does.  i write all of my manuals
in outline mode, and i definitely can't live without it.

  earlier today, i was expermienting with a friend with his
variation of psgml, and while it can fold and unfold elements
and lines, the fatal drawback is that if you unfold an element,
*everything* below that element is displayed.

  i want it so that, if i unfold or expand, say, a chapter,
i get only the elements immediately at the next level.  i can
then, of course, choose one of those to expand and so on.

  is there anything that will let me do this?  not surprisingly,
something free and open source would be nice but, heck, if i can 
find an editor to do what i want, i'm not averse to paying for
it.  within reason, of course.

  any suggestions?

rday